Electrostatic printing system



Aug. 24, 1965 c. o. CHILDRESS ELECTROSTATIC PRINTING SYSTEM 3 Sheets-Sheet 1 Filed Feb. 1, 1963 48 j 1 50B 48A& 485

K 'Lg u u uj 54 4 a so CL YDE O. CH/LDRESS INVENTOR.

BY fmtaziw Aug. 24, 1965 C. O- CHILDRESS ELECTROSTATIC PRINTING SYSTEM Filed Feb. 1, 1963 E E] E a 22 g Sheets-Sheet 2 C4 v05 0. CH/LDRESS INVENTOR.

A 77ORNEY United States Patent 3,2fi2difi2 ELECTRGSTATIC PRENTING SYSTEM Clyde 0. Chiidress, Palo Alto, alif., assignor to Electrostatic Printing Corporation of America, San Francisco, Caiih, a corporation of California Filed Feb. 1, 1963, Ser. No. 255,440 7 Claims. (Ci. 1tl1-If4) This invention relates to electrostatic printing systems and more particularly to improvements therein.

In an application for electrostatic printing by Clyde 0. Childress et al., Serial No. 12,714, filed March 4, 1960, now Patent No. 3,081,698 there is described an electrostatic printing process which effectively comprises transferring powder particles through an image forming electrode across an air gap in which an electric field has been established to an image receiving electrode, or to an article which is inserted between the two electrodes. The two electrodes may be in the form of parallel conductive planes that are held at an electrical potential difference so that they define a substantially uniform electrotastic field.

The image forming electrode may be a thin conductive member, such as a screen, in which all but the desired printing areas are masked. Printing is accomplished by bringing charged pigment particles into the electrostatic field through the image defining openings in the screen. Particles are transported along the lines of force perpendicular to the image plane and reproduce the patterns of openings of the image electrode on the receiving electrode or upon the article that has been interposed between the electrodes. The charged particles are firmly held to the surface upon which they are deposited by electrostatic attraction until the image is permanently fixed by heat, solvent vapor or other means depending upon the composition of the image particles.

It has been found that it is necessary to urge the pigment powder through the screen mesh openings using a brush or other means to cause the particles to enter into the electrostatic field which is established. Otherwise, despite the fact that the powder particles are smaller than those of the openings of the screen, powder does not pass freely and evenly through these openings. A rotating brush has been employed, which while effective, i somewhat messy, and the brush has to be replaced since it wears out with use.

An object of this invention is the provision of a novel arrangement in an electrostatic printing system for metering pigment powder through the openings in the printing screen.

Yet another object of the present invention is the provision of the unique and improved arrangement for moving pigment powder particles through the apertures in a stencil screen, which is employed in an electrostatic printing system.

Still another object of the present invention is the provision of an improved system for electrostatic printing wherein the brush employed for forcing powder particles through the stencil screen is eliminated.

These and other objects of this invention may be achieved in an arrangement wherein the powder particles, which are to be forced through the stencil screen having the apertured image to be printed, are mixed with magnetic particles such as iron filings, whose particle size is larger than those of the image screen openings. A magnetic field is provided for forcing the magnetizable particles toward the screen apertures when it is desired to perform the printing operation. The magnetic particles push or rub the pigment powder through the openings in the stencil screen whereupon they enter the electric field between the screen and the other electrode used for establishstencil screen and the other electrode used for establish- "ice ing the field, and are transported either to the other electrode or to an intercepting article, on which it is intended to print.

The novel features that are considered characteristic of this invention are set forth with particularity in the appended claims. The invention itself, both as to its organization and method of operation, as well as additional objects and advantages thereof, will best be understood from the following description when read in connection with the accompanying drawings, in which:

FIGURE 1 is an isometric view of an electrostatic printing system of the general type described in the aforementioned Childress et al. application shown to assist in understanding this invention.

FIGURE 2 is an isometric view of an embodiment of this invention.

FIGURE 3 shows an arrangement for mechanizing the embodiment of the invention.

FIGURE 4 shows an arrangement employing electromagnets for mechanizing the embodiment of the invention.

FIGURES 5, 6, and 7 are drawings of still other embodiments of the invention using opposing magnets, and

FIGURES 8 and 9 are respectively front and side views of yet another embodiment of the invention for obtaining particle acceleration.

Referring now to FIGURE 1 there is shown a view of an electrostatic printing system of the type described in the previously mentioned Childres et al. application This includes a container 10 which has one face thereof 12, in the form of an apertured mask wherein the apertures have the pattern of the character or figure desired to be printed. The mask is preferably made of a conductive screen wherein all of the screen is blocked off except for the pattern portions. The container 10 has therein, pigment powder particles of a size to be able to pass through the screen mesh in the open pattern areas.

An electric field is established by connecting a source of potential 14, between a conductive plate 16, and the conductive screen 1 2. Thus, the screen and plate serve as oppositely spaced electrodes for establishing and defining the field therebetween.

A pigment powder is contained in the container N. A rotating brush 18 rubs the pigment powder particles against the apertured portions of the screen 12, to cause them to enter the electric field which is established between the screen and the backplate 16. An article such as the sheet of paper 20, which is interposed between the screen and the backplate, receives the pigment powder particles in a pattern which is identical with the pattern of the screen. The paper 20 may be removed thereafter. The powder particles will adhere to the paper sheet and they can be fixed permanently thereto either by heat treatment or by a spray which forms a glaze thereover, or other suitable means. Where the article itself is sufficiently conductive to enable an electric field to be established therebetween and the conductive screen, then no backplate is necessary. A print or deposit of powder occurs on the conductive article substantially identically as would occur were a backplate to be used. While the description to be made hereafter refers to the use of the backplate, this should not be construed as a limitation upon the invention, since as pointed out, a separate backplate may be eliminated when an article is used having sufiicient conductivity to act as an electrode in establishing the electric field.

The pigment powder particles which are in the con tainer it), will not freely and uniformly of themselves, pass through the apertures which are open even though they are smaller in size than these apertures. Some arrangement must be used to meter them through these apertures. Thus far, one of the most effective mecha- 3 nisms for doing this has been to use a brush, which rubs against the screen apertures. Powder must be fed onto the brush which then rubs this through the screen apertures.

In accordance with this invention, as shown in FIG- URE 2, the brush may be eliminated. This is achieved by filling the container 22 with a mixture of the pigment particles 24, and a soft magnetizable material such as iron filings 26. The iron filings are selected to have a size which is larger than the screen apertures in the conductive screen 28. The backplate 30, may be spaced from the screen as before, and a source of potential 32 is employed to establish an electric field therebetween. The article 34, to be printed on, is shown in fragmentary form in order that the apertured image 28A may be exposed. A magnet 36 is moved reciprocally along the backplate 39 in order to lift the iron filings against the screen and more specifically the apertured image 28A. The side of the container 22 is shown as broken away, in order to indicate the type of action which occurs. The magnetic material 26 is lifted up by the attraction of the magnet 36 and in doing so forces powder through the openings in the image 28A. The pigment powder enters the electric field and is transported thereby onto the article 34 on which the writing is to occur.

The conductive plate 30 should be a material which does not act as a magnetic shield. Thus, a material such as aluminum, may be used.

As the magnet 36 is reciprocated back and forth along the backplate 30 the magnetic particles are picked up by the leading pole and dropped by the trading pole effectively causing a churning action which is beneficial to the powder mixtures since it serves to loosen any clumps or aggregates and brings fresh powder to the screen whereby the powder flow is made smooth. What is believed to happen is that the magnetic particles are picked up by the forward pole of the magnet and conveyed a short distance against the screen. The friction of the particles against the screen causes them to drop back to the following pole of the magnet and then to drop back into the containers as the forward pole picks up new material, continuously replenishing the mass held against the screen.

While the magnet 36 may be reciprocated manually, as shown in FIGURE 3, a plurality of magnets 40A through 40F may be mounted on a belt 42. The belt is rotatably supported by means of two cylinders 44, 46. These two spaced cylinders are rotatably driven by a motor (not shown) whereby the magnets successively pass along the backplate 30. This causes the magnetic particles to be alternatively attracted toward the screen 28 and then dropped back. Upon being attracted toward the screen the magnetic particles carry therewith powder particles and literally push these through the openings of the screen into the electric field created between the screen and the opposite electrode. The magnets 40A through 40F are successively passed across the backplate each time it is desired to effectuate an electrostatic printing operation.

FIGURE 4 is an electromagentic equivalent of FIG- URE 3. Instead of mechanically moving magnets past the backplate, a plurality of adjacent electromagnets are employed which are successively magnetized. A simple construction for such an arrangement is, as shown in FIG- URE 4, to employ a magnetic member 48 which has the shape of the field piece of an electric motor. This consists of a yoke 48A and a plurality of poles 43B extending therefrom toward the conductive screen. The yoke and poles may be made out of a plurality of laminations of suitable metal. They are coextensive with the conductive plate 30.

A separate winding 50A through 50E is wound over the yoke between each pair of poles 483. One end of each of these separate windings is connected together into a magnetizing potential source 52. The other end of each one of these separate windings is brought out to a separate contact terminal respectively 54A through 54E. A contact slider 56 is connected to the other side of the potential source 52. Motion of the slider from contact 54A through contact 54E successively energizes the successive electromagnets formed by the windings and the field piece 48. As a result, the magnetic material particles which are mixed with the powder in the container 22, are attracted toward the screen 28 carrying therewith the powder particles which are thereby forccd through the openings in said screen. Thus, the operation of the system shown in FIGURE 4 effectively produces substantially the same results as the system shown in FIGURE 3. The magnetic fields provided by the magnets in FIGURE 3 may be more sharply de fined than the saparate magnetic fields provided by the electromagnets in FIGURE 4. However, there is still an adequate attraction and churning action produced by the arrangement shown in FIGURE 4 to enable the magnetic particles to bring a fresh powder charge to the screen upon the successive magnetization of these electromagnets to produce a completely satisfactory operation.

For purposes of convenience it may be desirable to place the container with the conductive screen above the backplate. Such an arrangement is shown in FIGURE 5. The best churning action of the magnetic particles in the powder is secured by pulling the magnetic particles up into the parent mass of the pigment powder and away from the screen to which they are attracted by means of the magnet 60. The magnet 60 is moved reciprocally on the opposite side of the conductive plate 30 in the same manner as was described for the magnet 36 in FIG- URE 2. In conjunction with the magnet 60 two other magnets respectively 62 and 64 which are spaced apart by a nonmagnetic member 66, serve the function of lifting the magnetic material away from the screen at the bottom of the container 22 both before and after the magnetic field divide-d by the magnet 60 has pulled these particles down against the screen. The magnets 62 and 64 are made much weaker than the magnet 60 since they are closer to the magnetic particles than the magnet 60 and it is desired to keep the fields established by these magnets weak enough not to overcome the desired effects of the field established by the magnet 60. Accordingly, the spacing of the magnets 62 from 64 and the strength of these magnets is adjusted to insure that magnet 60 will pull the magnetic particles down against the screen, despite the presence of magnets 62 and 64.

Instead of having a pair of magnets 62, 64, which are moved in synchronism with the magnet 60, as shown in FIGURE 6, a single electromagnet may be employed. Electromagnet 70 is similar to the electromagnet 48 and has a winding 72 which is wound around the yoke of the magnet between the pole pieces. A source of electrical current 74 is connected to the winding 72. Electromagnet '70 is made sufficiently weak so that the magnet 60 which is moved reciprocally on the opposite side of the conductive plate 30 can overcome the effect of the magnetic field of the electromagnet and pull down the magnetic particles against the screen in the base of the container 22. These magnetic particles are thereafter pulled back into the powder mass by the effect of the weak magnetic field provided by the electromagnet 70. If desired, a switch 75, may be inserted in series with the source 74 and winding 72, which may be opened prior to a sweep of the magnet 69, and then may be closed, in order to achieve a maximum pulling effect by the field of magnet 60 on the magnetic particles in the container during the writing process.

As described thus far, the operation of the magnet has been a reciprocal one. That is, a magnetic field is successively moved across the screen to attract the magnetic particles successively toward the screen, thereby forcing powder particles through the screen into the electric field.

It is also possible to establish an instantaneous magnetic field over the entire surface area of the screen having apertured images desired to be printed. If this magnetic field is established quickly, more powder can be pushed through the apertures as a result of both the sudden acceleration of the magnetic particles and further, print ing occurs simultaneously over the entire pattern area instead of successively, which occurs with the moving field operation described. To efiectuate this two magnets may be employed, one on the side of the container and another one on the side of the conductive plate. The electromagnet on the side of the plate 30 can be connected when required, to a larger potential than the source 74 so that a larger current will flow through the winding and thereby a stronger field will be established than the field established by the magnet 70.

A better arrangement to accomplish this is shown in FIGURE 7. The magnet 70 establishes a weak field which pulls the magnetic particles in the container 22 toward the top of the container. When it is desired to print, then a switch 76 may be closed. This connects a source of potential 78 to a winding 80. The winding 80 is wound on a magnet arrangement 82, of the type shown in FIGURE 4. A large current is made to fiow in the winding 80, whereby a strong magnetic field is established which causes a rapid acceleration of the magnetic particles in the container 22 toward the screen, pushing ahead of them the powder particles through the apertures in the screen into the field established between the screen and the backplate 36. By using the switch 75 in a manner so that when switch 76 closes, switch 75 opens and vice versa, then the magnetic fields are alternately established. This reduces the amount of the current required to establish the printing magnetic field.

FIGURES 8 and 9 are respectively front and side views of an electrostatic printing arrangement, wherein the magnetic particles are advanced rapidly up to the screen to introduce pigment powder into the electric field through the open apertures of the screen and thereafter the magnetic particles are magnetically actuated away from the screen. A container 99 has a front face 92, which includes a mask with an opening in the form of the letter A. The backplate 94 is spaced from the mask and the medium 96 upon which printing is to occur is interposed between the front face and the backplate 94. An energizing source of potential 96, establishes an electric field between the conductive front face and the backplate.

A number of electromagnets respectively 98, 160, 192, are positioned adjacent one another so that the container is enclosed between their pole faces. Each one of the electromagnets has a winding, exemplified by the winding 104 shown in FIGURE 8. The winding has two output terminals respectively 106, 108. All of the output terminals 106 are connected to one side of the source of operating potential 11%. The selector switch 112 serves to connect each one of the terminals 108 in succession to the potential source 110. Thus, considering the selector switch 112 as being rapidly moved from right to left, in FIGURE 9, the magnets 98, 100, and 102 will be successively energized, successively advancing the magnetic particles toward the front face and bringing it up against the front face 92, whereby powder is forced through the openings in the mask to be injected into the electric field and thereafter deposited upon the article 95 upon which writing is to occur in the shape of the letter A. On the return of the selector switch to its off position, it retracts the magnetic particles away from the front face thereby churning the powder particles and allowing the interpositioning of more powder between the front face and the location of the magnetic particles.

By using a plurality of the structures shown in FIG- URES 8 and 9, a printer can be constructed having a different letter aperture in the front faces of each one of the containers 99. The letter desired to be printed is selected and then printed by selective actuation of one of the selector switches 112.

There has accordingly been described herein a novel, useful and improved electrostatic printing system wherein the transportation of powder particles through the conductive apertured screen into the electric field is effectuated magnetically.

I claim:

1. In an electrostatic printing system of the type wherein an electric field is established between two spaced opposing electrodes, one of said electrodes constituting a conductive apertured screen wherein the open apertures of said screen are arranged in a desired printing pattern, and wherein electroscopic pigment powder is introduced into said electric field through the apertures of said screen to be carried by said field onto an article on which printing is to occur, the improvement for introducing said powder into said field comprising a powder mixture including a substantially non-magnetic electroscopic powder wherein the powder particle size is smaller than the size of an aperture of said screen, and a plurality of magnetic carrier particles, the size of said magnetic carrier particles being larger than the apertures of said screen, container means for holding said mixture adjacent said screen, and means for producing motion of said carrier particles and powder for thrusting said carrier particles and powder against the apertures of said screen when it is desired to print comprising magnetic means for establishing a magnetic field, means for positioning said magnetic means at a side of said conductive screen for attracting said magnetic particles toward said conductive screen, and means for moving said magnetic means substantially parallel with the plane established by said conductive screen for insuring the introduction of powder and the motion of said magnetic particles over the entire area of said apertured printing screen.

2. In an electrostatic printing system of the type wherein an electric field is established between two electrodes, one of said electrodes constituting a conductive apertured screen wherein the open apertures of said screen are arranged in .a desired pattern, and wherein a substantially non-magnetic electroscopic pigment powder having a particle size smaller than that of an opening of said screen is introduced into said electric field through the apertures in said screen to be carried by said field to an article on which printing is to occur, the improved mechanism for introducing said powder into said field comprising a mixture including said electroscopic powder and a plurality of magnetic carrier particles, the size of said carrier particles being larger than the apertures of said screen, container means holding said mixture adjacent said screen, a first magnetic field producing means positioned on the side of said conductive apertured screen from which powder particles are to be moved into said electric field through said screen for attracting said magnetic particles away from said screen, and second means for establishing a magnetic field for directing said magnetic particles against said screen, said second means being positioned adjacent the side of said other of said two electrodes which does not confront said conductive apertured screen, said first magnetic means establishing a weaker magnetic field than said second magnetic means whereby said first magnetic means can attract said magnetic carrier particles away from said screen in the absence of the field of said second magnetic means, and said second magnetic means attracts said magnetic carrier particles toward said screen to thereby force powder particles through the openings of said screen into said electric field, and switch means for activating said second means for establishing a magnetic field when it is desired to print.

3. In an electrostatic printing system of the type wherein an article is to be printed on with electroscopic powder particles, said electrostatic printing system having means for establishing an electric field wherein said article is positioned, said means for establishing an electric field having as an element thereof a conductive apertured screen with the apertures arranged in the form of a pattern desired to be printed, said article to be printed on having one side opposite to and spaced from one surface of said screen, and said electrostatic printing system also including a container for said powder particles with said screen constituting one side thereof, the particles of said powder being smaller than the apertures of said screen and having to be pushed therethrough to be guided by said electric field to said article, the improvement for pushing said powder particles through said screen comprising a mixture within said container of a plurality of soft magnetic particles and electroscopic powder particles sufiicient to cover the pattern of apertures of said screen, said magnetic particles having a particle size larger than the apertures of said screen so as not to pass therethrough, and means positioned adjacent to the side of said article which is not opposite to said screen for magnetically attracting said magnetic particles toward said screen over the region of said pattern of apertures to push those powder particles which are between said magnetic particles and said screen through the apertures in said screen.

4. In an electrostatic printing system of the type Wherein an article is to be printed on with electroscopic powder particles, said electrostatic printing system having means for establishing an electric field wherein said article is positioned, said means for establishing an electric field having as an element thereof a conductive apertured screen with the apertures arranged in the form of a pattern desired to be printed, said article to be printed on having one side opposite to and spaced from one surface of said screen, and said electrostatic printing system also including a container for said powder particles with said screen constituting one side thereof, the particles of said powder being smaller than the apertures of said screen and having to be pushed therethrough to be guided by said electric field to said article, the improvement for pushing said powder particles through said screen comprising a mixture within said container of a plurality of soft magnetic particles and electroscopic powder particles sufficient to cover the pattern of apertures of said screen, said magnetic particles having a particle size larger than the apertures of said screen so as not to pass therethrough, a first magnetic field producing means positioned on the container side of said apertured screen actuatable for attracting said magnetic carrier particles away from said screen, second magnetic field producing means positioned adjacent to the side of said article which is not opposite to said screen actuatable for magnetically attracting said magnetic particles toward said screen over the region of said pattern of apertures to push those powder particles between said magnetic particles and said screen through the apertures in said screen, and switch means for actuating said second magnetic means and inactivating said first magnetic means when it is desired to print and for inactivating said second magnetic means and activating said first magnetic means when it is not desired to print.

5. In an electrostatic printing system of the type wherein an article is to be printed on with electroscopic powder particles, said electrostatic printing system having means for establishing an electric field wherein said article is positioned, said means for establishing an electric field including first and second spaced and parallel electrodes, said first electrode comprising a conductive apertured screen with the apertures arranged in the form of a pattern desired to be printed, said article to be printed on having one side opposite to and spaced from one surface of said screen, and said electrostatic printing system also including a container for said powder particles with said screen constituting one side thereof, the particles of said powder being smaller than the apertures of said screen and having to be pushed therethrough to be guided by said electric field to said article, the improvement for pushing said powder particles through said screen comprising a mixture within said container sufiicient to cover the pattern of apertures in said screen, a plurality of soft magnetic particles and electroscopic powder particles, said magnetic particles having a particle size larger than the apertures of said screen so as not to pass therethrough, and means for magnetically attracting said carrier particles to said screen for thrusting said carrier particles and powder against the apertures of said screen when it is desired to print comprising a plurality of magnets, means for supporting said plurality of magnets spaced from one another and adjacent the side of said second electrode which does not face said first electrode, and means for successively moving said means for supporting said plurality of magnets substantially parallel to and past the side of said second electrode which does not face said first electrode for successively attracting said magnetic particles toward said conductive screen to thrust the powder particles which are between said magnetic particles and said screen through said screen apertures.

'6. In an electrostatic printing system as recited in claim 5 wherein said means for supporting said plurality of magnets comprises an endless belt, and means for holding said magnets spaced along said endless belt.

'7. In an electrostatic printing system of the type wherein an article is to be printed on with electroscopic powder particles, said electrostatic printing system having means for establishing an electric field wherein said article is positioned, said means for establishing an electric field having a first and a second parallel, spaced electrode, said first electrode comprising a conductive apertured screen with the apertures arranged in the form of a pattern desired to be printed, said article to be printed on having one side opposite to and spaced from one surface of said screen, and said electrostatic printing system also including a container for said powder particles with said screen constituting one side thereof, the particles of said powder being smaller than the apertures of said screen and having to be pushed therethrough to be guided by said electric field to said article, the improvement for pushing said powder particles through said screen comprising a mixture within said container of soft magnetic particles and electroscopic powder particles sufiicient to cover the pattern of apertures of said screen, said magnetic particles having a particle size larger than the apertures of said screen so as not to pass therethrough, and means for attracting said magnetic particles to the apertures of said screen when it is desired to print comprising a plurality of inactive electromagnets, means for supporting said electromagnets adjacent to one another and parallel to the side of said second electrode which does not face said first electrode, and means for successively energizing said inactive electromagnets for attracting said magnetic particles toward said conductive screen to thrust powder particles between said magnetic particles and said screen through said screen apertures.

References Cited by the Examiner UNITED STATES PATENTS 2,787,556 4/57 Haas 1l717.5 2,874,063 2/59 Greig l1717.5 2,930,351 3/60 Giaimo 118637 3,081,698 3/63 Childress.

ROBERT E. PULFREY, Primary Examiner.

DAVID KLEIN, WILLIAM B. PENN, Excmzillers. 

3. IN AN ELECTROSTATIC PRINTING SYSTEM OF THE TYPE WHEREIN AN ARTICLE IS TO BE PRINTED ON WITH ELECTROSCOPIC POWDER PARTICLES, SAID ELECTROSTATIC PRINTING YSTEM HAVING MEANS FOR ESTABLISHING AN ELECTRIC FIELD WHEREIN SAID ARTICLE IS POSITIONED, SAID MEANS FOR ESTABLISHING AN ELECTRIC FIELD HAVING AS AN ELEMENT THEREOF A CONDUCTIVE APERTURED SCREEN WITH THE APERTURES ARRANGED IN THE FORM OF A PATTERN DESIRED TO BE PRINTED, SAID ARTICLES TO BE PRINTED ON HAVING ONE SIDE OPPOSITE TO AND SPACED FROM ONE SURFACE OF SAID SCREEN, AND SAID ELECTROSTATIC PRINTING SYSTEM ALSO INCLUDING A CONTAINER FOR SAID POWDER PARTICLES WITH SAID SCREEN CONSTITUTING ONE SIDE THEREOF, THE PARTICLES OF SAID POWDER BEING SMALLER THAN THE APERTURES OF SAID SCREEN AND HAVING TO BE PUSHED THERETHROUGH TO BE GUIDED BY SAID ELECTRIC FIELD TO SAID ARTICLE, THE IMPROVEMENT FOR PUSHING SAID POWDER PARTICLES THROUGH SAID SCREEN COMPRISING A MIXTURE WITHIN SAID CONTAINER OF A PLURALITY OF SOFT MAGNETIC PARTICLES AND ELECTROSCOPIC POWDER PARTICLES SUFFICIENT TO COVER THE PATTERN OF APERTURES OF SAID SCREEN, SAID MAGNETIC PARTICLES HAVING A PARTICLE SIZE LARGER THAN THE APERTURES OF SAID SCREEN SO AS NOT TO PASS THERETHROUGH, AND MEANS POSITIONED ADJACENT TO THE SIDE OF SAID ARTICLE WHICH IS NOT OPPOSITE TO SAID SCREEN FOR MAGNETICALLY ATTRACTING SAID MAGNETIC PARTICLES TOWARD SAID SCREEN OVER THE REGION OF SAID PATTERN OF APERTURES TO PUSH THOSE POWDER PARTICLES WHICH ARE BETWEEN SAID MAGNETIC PARTICLES AND SAID SCREEN THROUGH THE APERTURES IN SAID SCREEN. 